How deep and how close boreholes should go
Borehole depth sets how much pipe is in contact with the ground, and spacing sets how much neighbouring holes compete for the same heat. A house system keeps boreholes several metres apart, whereas the Drake Landing field packed 144 holes about 35 m deep at roughly 2.25 m spacing to store heat.
What sets borehole depth?
The designer starts with the heat the house needs on its coldest design day, then works out how many metres of loop can deliver that heat given the local rock or soil. Rock that conducts heat well needs less length than dry, loose sediment. Groundwater flow helps, because moving water carries heat in. Local rules and the drill rig also set practical limits.
Residential vertical boreholes are commonly on the order of tens of metres to over a hundred metres, and several shallower holes can substitute for one deep one. The deeper the hole, the steadier the temperature at the bottom, a point developed in ground temperature by depth.
Why does spacing matter?
Each borehole pulls heat from a cylinder of ground around it. If holes sit too close together, those cylinders overlap, the ground between them cools over the winter, and the loop fluid returns colder every year. That is thermal interference. The usual remedy is to space holes several metres apart so each has its own share of ground, or to add more length.
Spacing is a trade between land area and drilling length. Tighter spacing saves surface area but needs deeper or more holes to deliver the same output. Mixed-use systems that also reject heat in summer have a better chance of recovering, because the ground gets recharged.
How did Drake Landing do it differently?
The numbers are quite different from a house loop. The community's field held 144 boreholes at about 35 to 37 m depth and 2.25 m spacing, grouped inside a field around 35 m across. Close spacing was deliberate. The purpose was not to extract ambient ground heat but to build a warm block of earth by pushing summer solar heat into it, then draw that heat back out in winter. Interference between holes was the feature, since neighbours keeping each other warm reduces heat loss through the edges. Read more in borehole storage and borehole thermal energy storage.
What do the numbers look like in practice?
Take a simple hypothetical: a design requirement that works out to 600 m of total vertical loop. That could be four holes at 150 m, six at 100 m or ten at 60 m. Fewer, deeper holes mean less surface area but heavier drilling; more shallow ones suit smaller rigs and sites with shallow bedrock. The choice affects cost, as discussed in geothermal heat pump cost in Canada. The figures here are made-up round numbers to show the trade, not recommendations.
What should you ask a driller or designer?
Ask for the heat load calculation, the assumed ground conductivity, the borehole count and depth, the spacing and the predicted minimum loop fluid temperature after several winters. Ask whether a thermal response test was done on-site for larger systems. And ask how the choice compares with a trench layout, which is covered in horizontal vs vertical ground loops.
Why does the ground recover, and how fast?
After a winter of heat extraction, the ground around each borehole is cooler than before. During the off season, heat flows in from the surrounding earth, from groundwater and from the surface, and the temperature drifts back up. Whether the recovery is complete by next autumn depends on spacing, conductivity and how much heat was taken. Balanced systems that also dump heat into the ground in summer, through cooling, recover more fully.
An undersized field instead creeps colder year after year, losing efficiency slowly. That is why a competent design checks the long-run trend and why a homeowner is right to ask about it. For the broader cost and sizing story, see heat pump sizing in cold climates.
Quick answers
How deep are geothermal boreholes?
For houses they are commonly tens of metres to over a hundred metres, depending on heat load and ground conditions. The designer calculates total length first, then splits it into holes.
How far apart should boreholes be?
Typically several metres, so holes do not compete for the same heat. Closer spacing needs a deliberate design reason, as in heat storage.
What was the borehole layout at Drake Landing?
It had 144 boreholes about 35 m deep at roughly 2.25 m spacing, in a field around 35 m across. The tight pattern was chosen to store heat, not to harvest ambient ground heat.